Inactivated SIV or recombinant subunit vaccines have provided limited or no protection against SIV infection. Macaques vaccinated with live SIVMAC239 with a deletion in the nef gene (SIV fnef) resisted challenge with virulent SIV. However, SIV fnef persists indefinitely in macaques, provides limited or no protection until a year or more after immunization, and is pathogenic to neonates; these drawbacks limit its use as a vaccine. Previously, we demonstrated the attenuating and immune enhancing activities of interferon-gamma (IFN-G) in viral infections. We hypothesized that a vaccine of SIV FNEF expressing IFN-G would have enhanced safety and efficacy for macaques. We have generated SIVHYIFN, a replication-competent SIV FNEF that expresses human IFN-G. In this virus the expression of IFN-G is driven by the 5'LTR; two in-frame nef start codons were eliminated without altering the env amino acid sequence. SIVHYIFN is more stable, and expresses higher levels of IFN-G, than SIVSV-IFN (see vax11 report). We are conducting a comparative pathogenesis study in two groups of rhesus macaques intravenously inoculated with 104 TCID50 of SIV FNEF or SIVHYIFN. We have found that cell-associated virus loads were significantly lower for macaques inoculated with sivhyifn throughout the experiment. By week 2, when the first virus isolation was successful, only one animal (#26704) showed deletion of the IFN-G insertion; human IFN-G was present in the culture medium of the other five macaques co-cultures. In contrast, macaque #27047 was the only animal whose lymphocytes showed an intact, IFN-G-producing virus by week 6. By week 12, PCR amplification showed a 1KBP fragment, similar in size to the fragment yielded by wild type sivmac 239, for all animals. Macaques are going to be challenged with 100 AID50 of SIVMAC251 at 24 weeks after primary inoculation.